This invention relates to vacuum coating processes and devices. Specifically, the coating devices and processes allow for applying thin coating layers by means of so-called Direct Liquid Deposition on the surface of substrates. The term substrates is to be understood broadly: it may include but is not limited to flat or shaped (3-D) workpieces from semiconductor materials (e.g. wafer of circular or rectangular or irregular shape), plastic, ceramics, glass, metal or compounds thereof.
The invention has a wide spectrum of applications, ranging from depositing organic semiconductor (like OLED, Organic Photo-voltaic OPV, Organic Electronic) to depositing functional coatings (like anti-fouling, oleophobic, hydrophobic, protective or easy to clean treatments, e.g. for touch-panels). However, the invention is not limited to these applications.
Generically, Direct Liquid Deposition (DLD) processes fall within the broadest understanding of the family of deposition processes known as Physical Vapour Deposition (PVD) processes. In its simplest form, DLD comprises evaporating a liquid substance, such as a lubricant, a polymer or a polymer precursor material, in a heated vacuum recipient and causing at least one component of the substance to condense on the cooler surface of a substrate under vacuum which is at a temperature below the evaporation point of the substance at the pressure in the vacuum recipient. In the case of a polymer precursor material, this polymer precursor material may also polymerise on the surface, or later be cured e.g. by heat and/or humidity and/or ultraviolet light. The liquid is thus directly deposited upon the substrate surface with no chemical changes thereto: the only changes that take place are phase changes from liquid to vapour and back. Generically, the term “coating substance” is used in the present specification to refer to the substance at least one component of which is intended to be deposited on the substrate. “Precursor material” is used to refer to e.g. the coating substance dissolved in a solvent so as to create a solution.
Commonly, the coating substance to be deposited is extremely viscous, and thus difficult to handle. In consequence, the coating substance is dissolved as a solute in a solvent to create a liquid precursor material which has a significantly lower viscosity than the coating substance itself, which permits easy handling, dosing, and so on. Furthermore, in the case of the coating substance having the tendency to autopolymerise, the solvent helps to stabilise the coating substance and thereby prevent it from autopolymerising, and thereby thickening further which then requires a greater temperature for evaporation.
However, the presence of the solvent can affect the quality of the deposited coating. The solvent will have a boiling point significantly lower than that of the coating substance, which can cause splashing of the coating substance at the point of evaporation, in the worst-case leading to spots of liquid on the substrate. Furthermore, as the solvent evaporates progressively during deposition, the quality of the coating deposited can vary due to the composition of vaporised liquid substance/solvent in the vacuum recipient varying as one dose of precursor material evaporates.
US 2011/0195187 attempts to overcome some of these disadvantages in the context of applying oleophobic coatings to substrates on the basis of a liquid precursor material comprising a coating substance dissolved in a solvent. This document proposes a vaporising unit situated in the vacuum recipient itself. The liquid precursor material is fed into the vaporising unit, and is subjected first to an in-situ distillation step in which the solvent is evaporated out of the liquid precursor material under a first pressure and/or temperature regime, and a subsequent evaporation step in which the coating substance is evaporated under a second pressure and/or temperature regime. This is a slow, two-step process, and experimentation by the Applicant has shown the results of this process to be not entirely satisfactory, particularly with regard to the quality of the coating varying over the deposition time. Furthermore, since the process is slow, the coating substance is exposed to relatively high heat for a relatively long time, causing degradation of the substance, e.g. by autopolymerisation. This further reduces the quality of the coating.
US 2003/0175422 describes a vapour distribution arrangement for depositing lubricant films by Direct Liquid Deposition on the surfaces of computer hard disks. However, this arrangement has proven unsatisfactory for deposition of oleophobic coatings due to condensation of the material on parts of the nozzle, leading to a (partially) clogged nozzle, and uneven distribution of the coating.